There are several naturally occurring biological hazards that can threaten food safety. For clarity, these can be divided into microbiological threats, which include bacteria and viruses, and biological threats, which include multi-celled organisms such as parasites, moulds, fungi, and toxic plants. All these threats are ‘pathogenic’, meaning they all have the potential to cause illness or harm.
UK food poisoning data consistently shows that a small number of pathogens are responsible for most cases of illness. These include pathogenic bacteria such as Campylobacter and Clostridium, alongside pathogenic viruses such as Norovirus. As a result, this chapter focuses on microbiological threats, while addressing other biological hazards later.
Most pathogenic bacteria must be present in or on food in large numbers to overwhelm the body’s natural defences. However, some can cause illness at much lower levels by multiplying rapidly once inside the body.
The quantity of bacteria required to cause illness is known as the minimal infective dose (MID). This varies depending on factors such as age, health, and immune status. For example, some strains of E. coli can cause illness at very low doses in young children, while Listeria typically requires a higher number but presents serious risks to vulnerable groups.
Despite differences between individual types of bacteria, pathogenic food poisoning bacteria can be treated as a single category of risk at supervisory level because they share common characteristics:
Bacteria multiply when four conditions are present: food, moisture, warmth, and time. Removing just one of these factors prevents multiplication. In food operations, the most practical way to control bacterial growth without affecting taste, texture, or nutritional value, is denying them warmth and time. This is achieved through effective temperature control and by limiting the time food spends in conditions that allow bacteria to multiply.
Many pathogenic bacteria can also produce toxins during their lifecycle. Depending on the type, toxins may be released while the bacteria are alive or when the bacteria are destroyed by heat during cooking or reheating. Once toxins have formed, they are not destroyed by freezing, cooking, or reheating. This means food that has been held at unsafe temperatures for prolonged periods cannot be made safe simply by reheating. While heat may destroy the bacteria themselves, any toxins already present will remain.
These factors underpin several fundamental food safety rules, including the requirement that food may only be reheated only once, and that previously frozen food must not be refrozen. In all cases, food safety decisions must be based on time and temperature control rather than appearance, smell, or taste.
Some pathogenic bacteria are also capable of forming spores. When exposed to extreme heat, the bacterial cell breaks down and releases spores that can survive harsh conditions. When conditions later become favourable, either in food or within the human body, these spores can germinate into new bacteria that multiply rapidly, produce toxins and cause illness. Spore-forming bacteria such as Bacillus cereus and Clostridium perfringens are particularly associated with improper cooling and storage of cooked foods.
Most food poisoning incidents can be traced back to a small number of contamination routes. While pathogenic bacteria are often described as the cause of food poisoning, illness only occurs when controls fail. From a supervisory perspective, bacteria represent the hazard, but failures in process design, staff behaviour, or supervision represent the true causes.
Contamination may occur through direct human contact, through failures earlier in the supply chain, through breakdowns in food handling processes, or via the wider environment. Understanding how bacteria reach food allows you to block these routes before illness occurs.
| Contamination route | Where failure occurs | What actually goes wrong | Why supervision matters |
|---|---|---|---|
Human faecal - oral |
Personal hygiene |
Poor hand washing after toilet break allows pathogens from faeces to reach hands and mouth | Behaviour must be enforced consistently |
Exceptional supply chain contamination level |
Before delivery |
Food arrives already unsafe due to significant failures at slaughter, processing, or in transport | Requires rejection and escalation |
Human faecal - food - oral |
Process design and handling |
Contaminated hands or equipment transfer pathogens to food | Systems must block transfer |
External environment |
Access and segregation |
Pathogens enter via customers, equipment, waste, or pests | Requires layered controls |
It’s important to recognise that food safety risk increases cumulatively. Acceptable contamination, poor temperature control in transportation or within your premises, poor staff hygiene and delayed refrigeration may each seem minor in isolation but together compound to create conditions where food safety is at significant risk.
Although pathogenic bacteria are invisible, tasteless, and odourless, they are highly controllable through well-designed food safety systems. It is not realistic nor necessary to eliminate all bacteria from food, and acceptable legal microbiological limits exist for food produced or imported into the UK. Supervisory control therefore focuses on preventing growth and preventing new contamination once food enters the premises.
Processes that can be used to control pathogenic bacteria include:
In food service environments, temperature control remains the most widely used of these. All these processes must be supported by effective cleaning, segregation, and strong personal hygiene practices. Importantly, food safety systems must function reliably even when staff are busy, under pressure, or inexperienced.
Pathogenic viruses pose a significant food safety risk but behave very differently from pathogenic bacteria. Viruses are not living organisms and cannot multiply in food. Instead, they rely on people, food, and surfaces as vehicles of transmission. Once inside the body, viruses invade, mutate and damage human cells.
| Feature | Viruses | Bacteria |
|---|---|---|
Multiply in food |
No |
Yes |
Require food to survive |
No |
Yes |
Infective dose |
Very low |
High |
Spread via people |
Almost always |
Sometimes |
Primary control focus |
Exclusion and hygiene |
Multiplication prevention |
Norovirus is the most significant viral food safety threat in the UK. Very small numbers of virus particles are sufficient to cause infection, and outbreaks can spread rapidly in environments where people live or eat in close proximity. Symptoms closely resemble bacterial food poisoning and include vomiting, diarrhoea, and dehydration.
Viruses can survive on surfaces for extended periods and may spread through contaminated hands, shared surfaces, and airborne droplets produced during vomiting or coughing. Once an outbreak begins, it often affects multiple people and requires decisive leadership and significant effort to contain and break the cycle of infection.
If you suspect a Norovirus outbreak
Not all biological hazards are microbiological. Food safety can also be threatened by multi-celled organisms and toxic biological matter, including moulds, parasites, fungi, toxic animals or animal parts, and poisonous plants.